Oil sand deposits are found predominantly in the Middle East, Venezuela, and Western Canada. The term “oil sands” refers to large subterranean land forms composed of reservoir rock, water and heavy oil and/or bitumen. The Canadian bitumen deposits, being the largest in the world, are estimated to contain between 1.6 and 2.5 trillion barrels of oil. However, bitumen is a heavy, black oil which, due to its high viscosity, cannot readily be pumped from the ground like other crude oils. Therefore, alternate processing techniques must be used to extract the bitumen deposits from the oil sands, which remain a subject of active development in the field of practice. The basic principle of known extraction processes is to lower the viscosity of the bitumen, typically by the transfer of heat, to thereby promote flow of the bitumen material and recovery of same.
A variety of known extraction processes are commercially used to recover bitumen from oil deposits. Steam-Assisted Gravity Drainage, commonly referred to as SAGD, is one known method. A SAGD process is described, for example, in Canadian patent number 1,304,287. FIG. 1 is a representation of the subsurface arrangement of a typical prior art SAGD system 50. A boiler (not shown) on the surface supplies steam to steam injection piping 14 through connection 12. Steam is injected into subsurface formation 16 at intervals along the length of steam injection piping 14. The steam serves to heat subsurface formation 16, which reduces the viscosity of any hydrocarbons present in subsurface formation 16. Producer piping 18 is configured to accept the hydrocarbons where the hydrocarbons can be pumped to the surface through connection 20 for collection and processing.
The range of temperatures, and corresponding viscosities, required to achieve an economic flow rate is dependent on the hydraulic permeability of the reservoir in question. SAGD, as with most recovery strategies, is focused on increasing bitumen temperature within a limited region around a steam injection well. Once injected, the steam condenses within the bitumen deposit and its latent heat is transferred to the deposit by convection. The reduced-viscosity oil is then allowed to flow by gravity drainage to an underlying point of the reservoir, to be collected by a horizontal production well. The heavy oil/bitumen is then brought to the surface for further processing. Various pumping equipment and/or systems may be used in association with the production well.
Although effective, stand alone SAGD processes have several associated inefficiencies. First, the process is very energy intensive, requiring a great amount of energy for heating the volumes of water needed to generate the steam used for the heat transfer process. In addition, the amount of steam required is usually dictated by the need to maintain a certain pressure in the reservoir; this usually translates into a higher temperature than is optimally needed to mobilize the bitumen and, therefore, the expenditure of unnecessary energy. Further, as indicated above, upon releasing its heat to the formation, the injected steam condenses into water, which mixes with the mobilized bitumen and often leads to additional inefficiencies. For example, the water is generally recycled through boilers and, therefore, this requires costly de-oiling and softening processes/equipment. In addition, the original or initial separation of the bitumen and water requires further processing and costs associated with such procedures. Also, as common with other known active heating methods, significant energy input to the deposit is often transferred to neighboring geological structures and lost by way of conduction. Thus, the process becomes considerably energy intensive in order to achieve sufficient heating of the target formation.
SAGD operating temperature must be at the saturation temperature corresponding to the pore pressure in the reservoir, or the minimum temperature required for economic bitumen drainage rate, whichever is higher. Typical operating temperature is above 200 C. For the SAGD process, saturated steam at approximately 95 percent quality is injected, and saturated liquid water drains out the producer. As a result, neglecting piping and other losses, the ratio of heat delivered to the reservoir to heat required to produce the steam is
      Qres    Qsteam    =      xhfg          hf      -      ha      +      xhfg      Where
Qres is the heat delivered to the reservoir
Qsteam is the heat required to produce the steam
X is steam quality, typically 0.95 at the injection point
hf is the enthalpy of saturated liquid at the process temperature and pressure
hfg is the latent heat of vaporization
ha is the enthalpy of the water feed to the steam generator
The enthalpies vary with the saturation temperature and pressure. For 10% piping losses and a steam generator efficiency of 0.85, then the effective heat conversion efficiency (heat to reservoir divided by heat to steam generator) is 0.85, with heat recovery in both boiler blowdown and produced fluids. Field experience energy consumption for SAGD varies widely. SAGD performance is often measured in terms of SOR (steam oil ratio). As a point of reference for comparison with other processes, numerical predictions for energy consumption at the reservoir for SAGD under favorable conditions (uniform, isotropic hydraulic permeability, typical Athabasca bitumen, 30 m pay zone thickness) varies from 0.9 to 1.25 GJ/bbl heat at the reservoir per bbl bitumen produced. These correspond to SOR at the reservoir of 5 and 3, respectively
Dilution is another technique that has been used for the extraction of bitumen from oil sand or heavy oil deposits. The solvent based methods, such as VAPEX (vapor extraction), involve a dilution process wherein solvents, such as light alkanes or other relatively light hydrocarbons, are injected into a deposit to dilute the heavy oil or bitumen. This technique reduces the viscosity of the heavy hydrocarbon component, thereby facilitating recovery of the bitumen-solvent mixture that is mobilized throughout the reservoir. The injected solvent is produced along with bitumen material and some solvent can be recovered by further processing. Although solvent based methods avoid the costs associated with SAGD methods, the production rate of solvent based methods over the range of common in-situ temperatures and pressures has been found to be less than steam based processes. The solvent dilution methods also require processing facilities for the extraction of the injected solvent. Finally, these methods tend to accumulate material quantities of liquid solvent within the depleted part of the reservoir. Such solvents can only partially be recovered at the end of the process thereby representing an economically significant cost for the solvent inventory.
In order to understand the benefits of solvent processes, it is instructive to examine the basic phenomenology of gravity drainage, first developed and quantified for SAGD processes. A simplified representation of SAGD drainage is shown in FIG. 2.
In his landmark paper, Butler (1981) showed that SAGD drainage can be approximated by:
  Q  =                    2        ⁢                                  ⁢        ϕ        ⁢                                  ⁢        SoKg        ⁢                                  ⁢        α        ⁢                                  ⁢        Δ        ⁢                                  ⁢        H                    m        ⁢                                  ⁢                  υ          s                    Where
Q is the bitumen drainage volume per unit length of well per unit time
ϕ is porosity
So is oil saturation (noted by Butler as actually being change in oil saturation in the zone
K is effective permeability for oil flow (a fraction of the total permeability)
g is gravitational acceleration
α is the thermal diffusivity of the pay zone
ΔH is the gravitational head (distance from the top of the pay zone to the producer)
m is a dimensionless constant which is dependent upon the conditions used and upon the nature of the heavy oil (bitumen for SAGD applications), and
νs is the kinematic viscosity of the heavy oil (bitumen as in SAGD applications).
In current practice, flow predictions for given conditions are estimated using reservoir simulator codes that perform numerical analysis of the conditions. However, the driving parameters are as expressed explicitly in the Butler model above which clearly shows that drainage rate is inversely proportional to the square root of the kinematic viscosity. Butler also demonstrated via an energy balance that the rate of advance of the condensation line is governed by the thermal diffusivity of the material as shown in the equation. This represents an additional limitation on the maximum drainage rate of a SAGD process for a given viscosity. The addition of RF heating mitigates the thermal diffusivity rate limitation and thereby reduces the time required for reservoir drainage. Bitumen and heavy oil properties vary over a wide range, but all exhibit an extremely strong variation in viscosity with temperature as exemplified in FIG. 3.
One issue faced in known solvent extraction methods relates to a physical limitation. Bitumen deposits within the Alberta Athabasca region are too cold for the solvent to be commercially effective. At common reservoir temperatures, which are generally in the range of 10-15° C., the solvent dilution process is too slow to be economically viable. For a solvent extraction process to be effective, the bitumen deposit should preferably be at a threshold temperature of 40-70° C.
One solution to address the above problem has been to use steam as a heating means to render the solvent process more efficient. In this regard, a combination of SAGD and VAPEX methods has been proposed in order to combine the benefits of both while mitigating the respective drawbacks. Known as a solvent aided, or solvent assisted process, or SAP, this method involves the injection of both steam and a low molecular weight hydrocarbon into the formation. Gupta et al. (J. Can. Pet. Tech., 2007, 46(9), pp. 57-61) teach a SAP method, which comprises a SAGD process wherein a solvent is simultaneously injected into the formation with the steam. As indicated in this reference, a SAP process has been found to improve the economics of SAGD methods.
However, the above combination of steam and solvent processes has also been found to have disadvantages. As with typical SAGD processes, much of the heat contained in the steam is also lost to the rock and other material bounding the reservoir and is not retained by the bitumen itself. Thus, the energy efficiency of such method is low.
Another solution comprises the use of heated solvent being applied to the reservoir, such as with the N-SOLV™ process. The principle of this process being that the use of heated solvent may raise the temperature of the reservoir to the desired level for an effective dilution process. However, the vapor formed by heating the solvent has a low heat of vaporization, and therefore requires large volumes of solvent to be condensed during condensation to effectively raise the temperature of the bitumen.
Recently, as an alternative to the steam and solvent methods discussed above, another method of producing hydrocarbons from bitumen deposits involves the use of electromagnetic (EM) heating. In this method, one or more antennae are first inserted into the bitumen reservoir. A power transmitter is used to power the antennae, which induces an RF field through the reservoir. The absorbed RF energy heats the water and oil/bitumen within the reservoir, thereby resulting in flow of the hydrocarbon material. A production well is then used to withdraw the mobilized hydrocarbons, similar to the previously discussed methods. One example of an EM process is taught in U.S. Pat. No. 7,441,597, which teaches the use of EM heating to produce heavy oil from a reservoir. In such a process, an antenna is provided in a first horizontal well, and is powered to heat the surrounding heavy oil with RF energy. A second horizontal well is positioned below the first and is used as a production well into which the mobilized heavy oil flows. However, the EM heating method has been found to be very cost intensive, particularly due to the inefficiencies in transferring the generated power to the formation.
Electromagnetic heating uses one or more of three energy forms: electric currents, electric fields, and magnetic fields at radio frequencies. Depending on operating parameters, the heating mechanism may be resistive by Joule effect or dielectric by molecular moment. Resistive heating by Joule effect is often described as electric heating, where electric current flows through a resistive material. The electrical work provides the heat which may be reconciled according to the well known relationships of P=I2 R and Q=I2 R t. Dielectric heating occurs where polar molecules, such as water, change orientation when immersed in an electric field and dielectric heating occurs according to P=ωεr″ε0E2 and Q=ωεr″ε0E2 t, where P is the power density dissipated in the media, ω is the angular frequency, εr″ is the complex component of the material permittivity, ε0 is the permittivity constant of free space, E is the electric field strength, Q is the volumetric heat, and t is time. Magnetic fields also heat electrically conductive materials through the formation of eddy currents, which in turn heat resistively. Thus magnetic fields can provide resistive heating without conductive electrode contact.
Electromagnetic heating can use electrically conductive antennas to function as heating applicators. The antenna is a passive device that converts applied electrical current into oscillating electromagnetic fields, and electrical currents in the target material, without having to heat the structure to a specific threshold level. Preferred antenna shapes can be Euclidian geometries, such as lines and circles. Additional background information on dipole antennas can be found at S. K. Schelkunoff and H. T. Friis, Antennas: Theory and Practice, pp 229-244, 351-353 (Wiley New York 1952). The radiation pattern of an antenna can be calculated by taking the Fourier transform of the antenna's electric current flow. Modern techniques for antenna field characterization may employ digital computers and provide for precise RF heat mapping.
Antennas, including antennas for electromagnetic heat application, can provide multiple field zones which are determined by the radius from the antenna r and the electrical wavelength λ (lambda). Although there are several names for the zones they can be referred to as a near field zone, a middle field zone, and a far field zone. The near field zone can be within a radius r<λ/2π (r less than lambda over 2 pi) from the antenna, and it contains both magnetic and electric fields. The near field zone energies are useful for heating hydrocarbon deposits, and the antenna does not need to be in electrically conductive contact with the formation to form the near field heating energies. The middle field zone is of theoretical importance only. The far field zone occurs beyond r>λ/π (r greater than lambda over pi), is useful for heating hydrocarbon formations, and is especially useful for heating formations when the antenna is contained in a reservoir cavity. In the far field zone, radiation of radio waves occurs and the reservoir cavity walls may be at any distance from the antenna if sufficient energy is applied relative the heating area. Thus, reliable heating of underground formations is possible with radio frequency electromagnetic energy with antennas insulated from and spaced from the formation. The electrical wavelength may be calculated as λ=2π/β, where β=Im(γ), where Im(γ) indicates the imaginary component of γ, and γ=(jωμ(σ+jωε))1/2.
Where:
λ Is the wavelength;
β is the wavenumber;
γ is the phase propagation constant;
ω is the angular frequency;
μ is the magnetic permeability;
σ is the material conductivity; and
ε is the material permittivity.
Susceptors are materials that heat in the presence of RF energies. Salt water is a particularly good susceptor for electromagnetic heating; it can respond to all three RF energies: electric currents, electric fields, and magnetic fields. Oil sands and heavy oil formations commonly contain connate liquid water and salt in sufficient quantities to serve as an electromagnetic heating susceptor. “Connate” refers to liquids that were trapped in the pores of sedimentary rocks as they were deposited. For instance, in the Athabasca region of Canada and at 1 kHz frequency, rich oil sand (15 weight percent % bitumen) may have about 0.5-5% water by weight, an electrical conductivity of about 0.01 s/m, and a relative dielectric permittivity of about 120. As bitumen becomes mobile at or below the boiling point of water at reservoir conditions, liquid water may be a used as an electromagnetic heating susceptor during bitumen extraction, permitting well stimulation by the application of RF energy. In general, electromagnetic heating has superior penetration and heating rate compared to conductive heating in hydrocarbon formations. Electromagnetic heating may also have properties of thermal regulation because steam is not an electromagnetic heating susceptor. In other words, once the water is heated sufficiently to vaporize, it is no longer electrically conductive and is not further heated to any substantial degree by continued application of electrical energy.
Heating subsurface heavy oil bearing formations by prior RF systems has been inefficient due to traditional methods of matching the impedances of the power source (transmitter) and the heterogeneous material being heated, uneven heating resulting in unacceptable thermal gradients in heated material, inefficient spacing of electrodes/antennae, excessive electricity usage due to high process temperature, poor electrical coupling to the heated material, limited penetration of material to be heated by energy emitted by prior antennae and frequency of emissions due to antenna forms and frequencies used. Antennas used for prior RF heating of heavy oil in subsurface formations have typically been dipole antennas. U.S. Pat. Nos. 4,140,179 and 4,508,168 disclose dipole antennas positioned within subsurface heavy oil deposits to heat those deposits.
When RF heating is substituted for steam in an otherwise similar extraction process, the heat applied to the reservoir must be less than the SAGD reservoir heat, and the overall RF energy conversion process must be very efficient to achieve energy parity. This is driven by the energy loss associated with electric power generation (for a fossil fuel plant). For example, assume that an RF process requires 53% of the heat applied to the reservoir for the same flow rate as a SAGD process. Assume that system also converts 70% of the input electrical power to RF heat in the reservoir, and that the electric power is provided at 35% efficiency. That system would require 2.2 GJ of heat input to the power station to deliver the same amount of oil as the SAGD system delivering 1 GJ to the reservoir.
As discussed above, several methods are currently known for producing oil from bitumen reservoirs. The common element for all such known methods comprises the reduction in the viscosity of bitumen in the reservoir. Some methods, such as SAGD or N-SOLV™, involve the injection of heated media (water and solvent, respectively) as the heat source. The use of EM heating avoids the use of such heat delivering media. However, known electromagnetic heating methods are typically adapted to completely remove the requirement for any water or solvent from being used (see, for example, in U.S. Pat. No. 7,441,597). And as discussed above, each of these known methods involve several disadvantages, including a high cost.
The recovery of bitumen from reservoirs such as oil sands continues to be of interest particularly in view of the world's increasing energy demand. As such, the need to improve extraction efficiency of hydrocarbon containing reservoirs continues to gain importance. Despite the various prior art attempts discussed above, there exists a need for an efficient and cost-effective method for in situ recovery of bitumen and/or heavy oil from underground reservoirs.
The present system, described herein, stands unique in providing a method wherein EM heating is used initially as a pre-conditioning phase, not to result in production of oil but to increase the temperature of the bitumen, at least within a defined region, to a level where solvent vapor can be used as the final production medium. The solvent achieves this goal by diluting the pre-conditioned, i.e. pre-heated, bitumen and results in mobility thereof into a production well.
The following references are provided are related to the present subject matter. The entire contents of all references listed in the present specification, including the following documents, are incorporated herein by reference.    Butler, R. M. “Theoretical Studies on the Gravity Drainage of Heavy Oil During In-Situ Steam Heating”, Can J. Chem Eng, Vol 59, 1981